Accelerometers are used to detect motion in one or more directions. MEMS technology has been leveraged to manufacture many different types of sensors and actuators, including accelerometers. For example, these devices have become commercially important to deploy airbags in vehicles. Miniaturization of these devices offers the potential for lower cost, higher throughput, and higher sensitivity.
FIG. 1 illustrates operation of a prior art z-axis MEMS accelerometer 100 at time points 100a and 100b. The prior art system shown in FIG. 1 incorporates a pendulous design in the respect that accelerometer 100 includes a proof mass 102 designed to swing like a pendulum to detect motion. The pendulous motion results from having a proof mass 102 tethered to an anchor 110 via a single flexure 104 in microchannel 108. Electrodes 106a and 106b are situated both above and below proof mass 102 and serve to sense capacitances C1 and C2 and to drive proof mass 102 to a predetermined position along the z axis in microchannel 108, as shown at time point 100a. 
Flexure 104 acts as a spring with spring constant k to allow proof mass 102 to move in the +z or −z direction in response to an acceleration experienced by accelerometer 100. For example, if accelerometer 100 experiences an acceleration in the +z direction, proof mass 102 will move in the −z direction. Conversely, if accelerometer 100 experiences an acceleration in the −z direction, proof mass 102 will move in the +z direction.
At time point 100a, proof mass 102 is in its null position (i.e., the position it finds itself in when MEMS accelerometer 100 experiences no acceleration). Proof mass 102 is typically made out of a conductive material and thus, at time point 100a, two capacitors are formed with capacitances C1 and C2 between the proof mass 102 and electrodes 106a, 106b, respectively. At time point 100b, accelerometer 100 has experienced an acceleration in the −z direction, thereby causing proof mass 102 to swing up in the +z direction. Due to displacement of proof mass 102 at time 100b, capacitances C1 and C2 have changed in value. In this particular scenario, capacitance C1 has increased to C1* and capacitance C2 has decreased to C2*.
Acceleration experienced by prior art accelerometer 100 is detected through a differential capacitance measurement. In particular, at time point 100a, this differential capacitance measurement is C1-C2, and at time point 100b, C1*-C2*. This change in differential capacitance is sensed through electronics connected to MEMS accelerometer 100.
Prior art pendulous MEMS accelerometer designs, such as the one shown in FIG. 1, can be operated with both open loop and closed loop control. In open loop operation, accelerometer 100 senses a differential capacitance change due to passive displacement of proof mass 102. In closed loop operation, a voltage applied at electrodes 106a, 106b is used to drive proof mass 102 to a “null position” in microchannel 108 (e.g., the center of channel 108 along the z axis), thereby acting as an “electrical spring” superimposed on the mechanical flexure or spring 104. Any external acceleration experienced by device 100 then causes proof mass 102 to move away from its null position and upon detection of this displacement by electrodes 106a, 106b, the applied voltage is adjusted to bring proof mass 102 back to its null position.
The sensor signal in pendulous designs, such as the differential capacitance signal in the design shown in FIG. 1, is ideally insensitive to cross-axis acceleration, i.e., to acceleration orthogonal to the axis of flexure. This is difficult to achieve in practice, however. In FIG. 1, for example, movement of proof mass 102 purely in the x direction will not generate a sensor signal when the center of mass of proof mass 102 is located at z=0. In this instance, flexure 104 will be in tension or compression for acceleration in the x or −x direction. However, if the center of mass of proof mass 102 is displaced from z=0, a cross-axis acceleration (i.e., x or y acceleration) will cause rotation that is sensed by system 100, giving rise to a sensor signal. Thus, the differential capacitance measurement C1-C2 would change even though proof mass 102 was not experiencing acceleration along the z axis (i.e., the direction for which it was designed to detect acceleration).
Moreover, mechanical springs are much more susceptible than electrical springs to environmental changes such as temperature variations. The response of prior art MEMS accelerometer systems can be significantly affected by the undesirable mechanical properties of flexures 104. More specifically, when operating device 100 in closed loop mode, drive electrodes 106a, 106b pull proof mass 102 back to its null position, but the free body diagram for proof mass 102 along the z axis will be a summation of the mechanical forces due to the mechanical spring along the z axis (i.e., flexure 104) and the electrostatic forces due to the voltage applied between drive electrodes 106a, 106b. Because the mechanical properties of flexure 104 can play a significant role in the mechanical response of proof mass 102, prior art accelerometers 100 can also suffer from significant stability issues with respect to environmental changes such as temperature variations.